This invention relates generally to telecommunications and more particularly to radio receivers used in mobile radiotelephone systems.
UHF and VHF mobile radiotelephone systems are used extensively throughout North America. Conventional two-way communications in these systems takes place between a mobile radiotelephone subscriber station and a central base station. Unlike the mobile station, the base station has the capability of simultaneous reception and transmission on all assigned frequency channels. The base station may also have several high power transmitters separated geographically. The use of geographically separated transmitters generally serves two purposes: it expands the system service area over a single transmitter location and it reduces the signal interference caused by signal blocking landmarks. It has been found, however, that such multiple transmitters can also cause problems for the mobile receivers.
Conventional mobile receivers automatically select the first idle frequency channel found in scanning the assigned frequency channels. If all frequency channels are located at a single base station transmitter, any frequency channel is as good or as poor as any other, so this selection procedure is more than adequate. But because signal strengths vary within a service area when multiple base transmitters are used, conventional mobile receivers will not necessarily select the best available frequency channel. And in the worst case, a conventional receiver can pick a frequency channel that is so marginal that the subscriber is left without service. This situation occurs when the mobile receiver selects a frequency channel that has sufficient idle tone present on the carrier signal for the receiver to recognize the channel as available. But due to non-reciprocity of carrier signals, the mobile transmitter is not able to access the terminal, and functionally the subscriber finds he is unable to communicate with the base.
It is an object of this invention to prevent a service interruption for a mobile subscriber due to a mobile receiver which selects marginal frequency channels.
It is a general object of this invention to provide apparatus to enable a mobile receiver to select the best available frequency channel wherever the subscriber happens to be located within the service area.